1. Field of the Invention
The present invention generally relates to image forming apparatuses and optical scanning apparatuses and, more particularly, to an image forming apparatus such as a copy machine, a printer, a facsimile machine, a plotter or a combination machine of the aforementioned and an optical scanning apparatus used in such an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus using a Carlzon process, latent-image formation, development and transfer are performed in association with rotation of a photosensitive drum. Accordingly, in a multiple-color image forming apparatus in which a plurality of photosensitive drums are arranged in a direction of conveying a transfer medium so as to overlap toner images formed by image forming stations of each color, unless an irradiating position of a light beam on each of the photosensitive drums is accurately aligned, a color shift or a color change may be generated, which causes deterioration of image quality.
Thus, as disclosed in Japanese Utility Model Publication No. 2536711 and Japanese Laid-Open Patent Application No. 2002-127497, light beams emitted by light sources corresponding to each color are scanned by being deflected by a single polygon scanner and a plurality of folding mirrors are provided so as to guide the light beams to respective photosensitive drums, and those components parts are integrally supported on a common housing so as to maintain positional accuracy between scan lines of each color. Thereby, it is configured and arranged to prevent a relative displacement with respect to irradiating positions of the light beams on each of the photosensitive drums.
Additionally, Japanese Laid-Open Patent Applications No. 2002-169353 and No. 2003-195206 disclose examples of arranging optical units for each color. In this those cases, similar to the above, positional accuracy between scanning lines of each color is maintained by positioning and supporting the optical units to a common side-plate frame.
As mentioned above, in a multiple-color image forming apparatus in which a plurality of image forming stations (hereinafter, simply referred to as stations) are arranged in a conveyance direction of a transfer member, such as a transfer belt or a conveyance belt for conveying recording papers so as to overlap colors, unless registration positions of toner images of the stations, which are transferred from the photosensitive drums to the transfer member, accurately overlap with each other, it causes a color displacement or a color change. Thus, in an optical scanning apparatus, the housing to which the image forming components including image forming stations are mounted is formed by a metal material such as an aluminum die-cast alloy.
However, there is a problem in that productivity is low and a cost is high since it takes time and labor to machine receiving surfaces of component parts. Thus, a plastic housing such as disclosed in Japanese Laid-Open Patent Application No. 2002-127497 has been used. However, the plastic housing is inferior to a metal housing in reliability over a long time. Since a stress is generated in the housing due to an environmental change, the above-mentioned plurality of folding mirrors tend to change in their relative positions and angles. Accordingly, an irradiating position on each of the photosensitive drums is displaced and a time taken from an irradiating position to a transfer position in each station, which causes a problem of occurrence of a registration displacement.
Thus, in a case where a predetermined temperature raise occurs or a number of continuously printed sheets exceeds a predetermined number, a job is interrupted to detect a registration displacement so as to readjust writing timings between the stations.
In order to detect the registration displacement, a detection pattern must be recorded on a transfer member. Thus, during such a time period, it is set to a recording unable state, which elongates print wait time and results in deterioration in operation. Additionally, if a number of corrections is increased, an amount of consumption of the waist toner is increased. Thus, it is desirous to avoid the correction from being frequently performed.
Especially, if a distance between the stations is increased, the housing is enlarged due to thermal expansion, and, thereby, a warp tends to be generated in the housing, which results in difficulty in acquiring dimensional stability. Additionally, since a thickness is increased inevitably and a mixing ratio of glass fibers is increased, productivity is deteriorated and a cost is increased even if a plastic material is used.
Additionally, since the housing becomes weak to a vibration in a direction perpendicular to a bottom surface, which propagates from a polygon motor or an external part, it is preferable to reduce the size of the housing as small as possible.
On the other hand, there is a method of raising rigidity using a metal sheet as disclosed in Japanese Laid-Open Patent Application No. 2002-311369. However, if an entire housing is made of a metal sheet, the configuration of the metal housing becomes complex to support a plurality of light sources and scanning lenses of the plurality of stations. Thus, there is a problem in that a number of fabrication processes is increased and assembling becomes troublesome, which results in an increase in cost.
Japanese Laid-Open Patent Applications No. 2002-169353 and No. 2003-195206 disclose examples of accommodating component parts in a plastic housing and position and support the component parts to a common metal sheet frame that constitutes a structural body of the apparatus body. However, there is a problem in that a positional accuracy cannot be maintained due to a stress applied to the framed is the installation surface of the apparatus body is not flat.